1. Field of the Invention
The present invention relates to a probe scanning mechanism for a scanning probe microscope capable of observing samples at a resolution of an atomic order.
2. Description of the Related Art
Scanning probe microscopes (SPMs) are conventionally known as apparatuses for observing samples at a resolution of an atomic order.
A scanning tunneling microscope (STM) invented by Binnig, Rohrer, et al. is one type of SPM, and an atomic force microscope (AFM) is another type of SPM. The AFM is an apparatus that utilizes the elemental technologies of the STMs, including servo technology, and can be used to observe insulating samples at a resolution of an atomic order.
The AFM is provided with a cantilever, which has a pointed projection (probe) on its free end. When the probe is brought close to a sample, the free end of the cantilever is displaced by an interactive force (atomic force) between the atoms of the distal end of the probe and those of the sample surface. Information relating to the irregularity of the sample surface and the like can be obtained in a three-dimensional fashion by scanning the probe with respect to the sample surface in the XY-direction, while measuring the displacement of the free end of the cantilever electrically or optically.
Available scanning mechanisms include an XY-stage, the so-called tripod, the so-called piezoelectric tube scanner, etc. The XY-stage comprises X- and Y-tables, which are movable at right angles to each other, and layer-built piezo-actuators for moving the tables. The tripod is formed of three layer-built piezo-actuators that are connected at right angles to one another. The piezoelectric tube scanner comprises a cylindrical piezoelectric element, a continuous common circumferential electrode on the inner surface of the tube, and four circumferentially divided driving electrodes on the outer surface of the tube.
The X-direction motion of the X-table and the Y-direction motion of the Y-table, in the XY-stage, are completely independent of each other, so that one of these motions cannot be influenced by the other. Since the inertial mass of each table is substantial, however, the XY-stage is not suited for high-speed scanning.
The tripod and the tube scanner can perform a scanning operation at a relatively high speed. Since a motion in one direction is dependent on a motion in another, however, the former inevitably influences the latter.
Thus, in the tripod and piezoelectric tube scanner, the scanning frequency is restricted to 300 Hz in consideration of the natural frequency of each piezo-actuator and ringing. In actual practice, the scanning frequency for the tube scanner is several tens of hertz at the most.
The above-mentioned ringing is the vibration that is left after the occurrence of the displacement when voltage applied to the piezo-actuators is increased suddenly. In order to obtain a great scanning width, the piezoelectric tube scanner requires a substantial overall length. The longer the scanner, however, the more easily the ringing occurs.
The above described scanning mechanisms can be used without any substantial hindrance in observing stationary samples or samples that slowly change with time. Since the scanning cycle is long, however, such scanning mechanisms cannot enable observation of the way phagocytes reach out their pseudopodia or cells react to stimuli, for example.